Volkswagen Cars 2000-05

Evaporative Emission Control (EEC) Systems

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Gasoline fuel is a major source of pollution, before and after it is burned in the automobile engine. From the time the fuel is refined, stored, pumped and transported, again stored until it is pumped into the fuel tank of the vehicle, the gasoline gives off unburned hydrocarbons (HC) into the atmosphere. Through the redesign of storage areas and venting systems, the pollution factor was diminished, but not eliminated, from the refinery standpoint. However, the automobile still remained the primary source of vaporized, unburned hydrocarbon (HC) emissions.

Fuel pumped from an underground storage tank is cool but when exposed to a warmer ambient temperature, will expand. Before controls were mandated, an owner might fill the fuel tank with fuel from an underground storage tank and park the vehicle for some time in warm area, such as a parking lot. As the fuel would warm, it would expand and should no provisions or area be provided for the expansion, the fuel would spill out of the filler neck and onto the ground, causing hydrocarbon (HC) pollution and creating a severe fire hazard. To correct this condition, the vehicle manufacturers added overflow plumbing and/or gasoline tanks with built in expansion areas or domes.

However, this did not control the fuel vapor emission from the fuel tank. It was determined that most of the fuel evaporation occurred when the vehicle was stationary and the engine not operating. Most vehicles carry 5-25 gallons (19-95 liters) of gasoline. Should a large concentration of vehicles be parked in one area, such as a large parking lot, excessive fuel vapor emissions would take place, increasing as the temperature increases.

To prevent the vapor emission from escaping into the atmosphere, the fuel systems were designed to trap the vapors while the vehicle is stationary, by sealing the system from the atmosphere. A storage system is used to collect and hold the fuel vapors from the carburetor (if equipped) and the fuel tank when the engine is not operating. When the engine is started, the storage system is then purged of the fuel vapors, which are drawn into the engine and burned with the air/fuel mixture.

Gasoline fuel is a major source of pollution, before and after it is burned in the automobile engine. From the time the fuel is refined, stored, pumped and transported, again stored until it is pumped into the fuel tank of the vehicle, the gasoline gives off unburned hydrocarbons (HC) into the atmosphere. Through the redesign of storage areas and venting systems, the pollution factor was diminished, but not eliminated, from the refinery standpoint. However, the automobile still remained the primary source of vaporized, unburned hydrocarbon (HC) emissions.

Fuel pumped from an underground storage tank is cool but when exposed to a warmer ambient temperature, will expand. Before controls were mandated, an owner might fill the fuel tank with fuel from an underground storage tank and park the vehicle for some time in warm area, such as a parking lot. As the fuel would warm, it would expand and should no provisions or area be provided for the expansion, the fuel would spill out of the filler neck and onto the ground, causing hydrocarbon (HC) pollution and creating a severe fire hazard. To correct this condition, the vehicle manufacturers added overflow plumbing and/or gasoline tanks with built in expansion areas or domes.

However, this did not control the fuel vapor emission from the fuel tank. It was determined that most of the fuel evaporation occurred when the vehicle was stationary and the engine not operating. Most vehicles carry 5-25 gallons (19-95 liters) of gasoline. Should a large concentration of vehicles be parked in one area, such as a large parking lot, excessive fuel vapor emissions would take place, increasing as the temperature increases.

To prevent the vapor emission from escaping into the atmosphere, the fuel systems were designed to trap the vapors while the vehicle is stationary, by sealing the system from the atmosphere. A storage system is used to collect and hold the fuel vapors from the carburetor (if equipped) and the fuel tank when the engine is not operating. When the engine is started, the storage system is then purged of the fuel vapors, which are drawn into the engine and burned with the air/fuel mixture.

Operation



This system prevents the escape of raw fuel vapors (unburned hydrocarbons, or HC) into the atmosphere. When the engine is not running, fuel vapors that build up in the tank flow through a hose to a carbon canister located in the inner fender below the air cleaner. When the engine is running, the vapors in the canister are carried to the intake manifold and burned in the engine.

On vehicles with a Digifant engine management system, the evaporative emission system is completely vacuum operated. The canister is purged of fuel vapors when a vacuum actuated valve is opened and fresh air is drawn into the open bottom of the canister. Vacuum for the valve is generated only at partial throttle openings. The purge air flows to the intake manifold through an orifice that limits the flow. These two features avoid radical air/fuel mixture changes when the canister is purged.

On vehicles with CIS-E fuel injection, the carbon canister is isolated from the intake manifold by a solenoid valve and a frequency valve. The solenoid valve is ON (open) whenever the engine is running. The frequency valve is cycled open and closed by the Motronic control unit to control flow rate depending on coolant temperature and engine speed/load conditions.

On both systems, the vacuum line to the canister connects to the vacuum vent valve. This valve is mounted near the fuel filler and allows vapors to flow to the canister but will close if the vehicle turns over to prevent a liquid fuel leak.

The system does not require any service under normal conditions other than to check for leaks. Check the hoses visually for cracks and check the seal on the gas tank filler cap. Replace the cap if the seal is split. If any hoses are in need of replacement, use only hoses marked EVAP, available from your local automotive supply store. If there is a strong smell of raw fuel from under the hood when the engine is not running, test the purge control valve.

Changes in atmospheric temperature cause fuel tanks to breathe, that is, the air within the tank expands and contracts with outside temperature changes. If an unsealed system was used, when the temperature rises, air would escape through the tank vent tube or the vent in the tank cap. The air that escapes contains gasoline vapors.

The Evaporative Emission Control System provides a sealed fuel system with the capability to store and condense fuel vapors. When the fuel evaporates in the fuel tank, the vapor passes through the EVAP emission valve, through vent hoses or tubes to a carbon filled evaporative canister. When the engine is operating the vapors are drawn into the intake manifold and burned during combustion.

A sealed, maintenance free evaporative canister is used. The canister is filled with granules of an activated carbon mixture. The charcoal granules absorb fuel vapors entering the canister. A vent cap is located on the top of the canister to provide fresh air to the canister when it is being purged. The vent cap opens to provide fresh air into the canister, which circulates through the charcoal, releasing trapped vapors and carrying them to the engine to be burned.

Fuel tank pressure vents fuel vapors into the canister. They are held in the canister until they can be drawn into the intake manifold. The canister purge valve allows the canister to be purged at a pre-determined time and engine operating conditions.

The canister purge valve controls vacuum to the canister. The valve is operated by the Engine Control Module (ECM). The ECM regulates the valve by switching the ground circuit on and off based on engine operating conditions. When energized, the valve prevents vacuum from reaching the canister. When not energized the valve allows vacuum to purge the vapors from the canister.

During warm up and for a specified time during hot starts, the PCM energizes the valve; preventing vacuum from reaching the canister. The EVAP purge control solenoid begins to operate when the engine coolant temperature reaches a predetermined operating temperature.

Once the proper coolant temperature is achieved, the PCM controls the ground circuit to the valve as necessary. When the PCM opens the ground, this allows vacuum to flow through the canister and vapors are purged from the canister into the throttle body. During certain idle conditions, the PCM may energize the purge valve to control fuel mixture calibrations.

The fuel tank is sealed with a pressure-vacuum relief filler cap. The relief valve in the cap is a safety feature, preventing excessive pressure or vacuum in the fuel tank. If the cap is malfunctioning, and needs to be replaced, ensure that the replacement is the identical cap to ensure correct system operation.

The following components are part of and affect the operation of the EVAP (Evaporative Emission) Control system:



Fuel Tank
 
Fuel Fill Cap
 
Evap Purge Canister
 
Fuel Tank Pressure sensor
 
Engine Control Module (ECM)
 
EVAP Purge Control Solenoid Valve
 

The evaporative control system does not require any service under normal conditions other than to check for leaks. Check the hoses visually for cracks, breaks, etc. Also check the seal on the gas tank filler cap. Replace the cap if the is split. If any hoses are in need of replacement, use only hoses marked EVAP, available from your local automotive supply store. If there is a strong smell of raw fuel from under the hood when the engine is not running, test the purge control valve.

The Evaporative Emission (EVAP) Controls are monitored by the Engine Control Module (ECM) and if found to be malfunctioning the ECM records the problem in the fault memory as a Diagnostic Trouble Code (DTC). If the problem persists or compromises the vehicle's emissions, the Check Engine Light/Malfunction Indicator Light (MIL) will be activated.

The system is considered to be malfunctioning when:



No purge airflow can be detected
 

Before the Evaporative System can be tested, no fault conditions must be met for the following:



Vehicle speed,
 
Engine coolant and/or intake air temperature
 
Engine idle speed
 
Oxygen sensors
 
Purge valve
 
Fuel tank pressure sensor
 
Evaporative emission canister shut off valve
 
Throttle position
 

  1. Start the engine and allow it to warm. The engine coolant temperature must be at least 176 °F (80°C), at an altitude of less than 8,202 feet (2502 m) and the engine operated for at least 6 minutes since the start of drive cycle.
  2.  
  3. Connect a suitable Data scan tool (DST) to the 16-pin Data Link Connector (DLC) and check for faults.
  4.  



Click image to see an enlarged view

Fig. Evaporative canister frequency valve and control valve system used 2.0L 16V engines



Click image to see an enlarged view

Fig. The evaporative canister is not a regular service item, and can be tucked out of the way. This one is located in an under-body cavity below the trunk-1.8L Passat shown

Removal & Installation



  1. Disconnect the electrical connector.
  2.  
  3. Loosen and remove the hose clamps and remove the valve from the EVAP hoses and remove the valve.
  4.  
  5. Installation is in reverse order of removal.
  6.  

Service





Click image to see an enlarged view

Fig. Evaporative canister and purge valve used with Digifant engines



Click image to see an enlarged view

Fig. Evaporative canister frequency valve and control valve system used 2.0L 16v engines



Click image to see an enlarged view

Fig. Evaporative system used on A3 vehicles

All vehicles with gasoline engines are equipped with some form of fuel vapor control device. The evaporative emission control system prevents the escape of raw fuel vapors (unburned hydrocarbons, or HC) into the atmosphere. On Volkswagens, a carbon (charcoal) canister is used to store fuel tank vapors that accumulate when the engine is not running. When the engine is running, the vapors in the canister are carried to the intake manifold by allowing fresh air into the bottom of the canister.

Other components of the system include a non-vented fuel filler cap, fuel tank expansion chamber and one or more check valves to prevent liquid fuel from entering the canister.

Early models use a vacuum-operated valve to vent the fuel vapors into the intake tract; later models use an electric valve actuated by the Electronic Control Module (ECM).

On all Fox models, the evaporative canister is located in the engine compartment, under the master cylinder. On all other models, the evaporative canister is located on the passenger side front wheel housing.

The evaporative control system does not require any service under normal conditions other than to check for leaks. Check the hoses visually for cracks, breaks, etc. Also check the seal on the gas tank filler cap. Replace the cap if the is split. If any hoses are in need of replacement, use only hoses marked EVAP, available from your local automotive supply store. If you suspect a problem with the purge valve or any other component, see Section 5 for information on testing the system.

Testing





Click image to see an enlarged view

Fig. Evaporative canister frequency valve and control valve system used 2.0L 16v engines



Click image to see an enlarged view

Fig. Evaporative system used on early VW models



Click image to see an enlarged view

Fig. The EVAP purge valve is mounted on the air filter housing on 1.8L models

All vehicles with gasoline engines are equipped with some form of fuel vapor control device. The evaporative emission control system prevents the escape of raw fuel vapors (unburned hydrocarbons, or HC) into the atmosphere. A carbon (charcoal) canister is used to store fuel tank vapors that accumulate when the engine is not running. When the engine is running, the vapors in the canister are carried to the intake manifold by allowing fresh air into the bottom of the canister.

Other components of the system include a non-vented fuel filler cap, fuel tank expansion chamber and one or more check valves to prevent liquid fuel from entering the canister.

Early models use a vacuum-operated valve to vent the fuel vapors into the intake tract; later models use an electric valve actuated by the Electronic Control Module (ECM).

The evaporative canister is located in the engine compartment.

The evaporative control system does not require any service under normal conditions other than to check for leaks. Check the hoses visually for cracks, breaks, etc. Also check the seal on the gas tank filler cap. Replace the cap if the is split. If any hoses are in need of replacement, use only hoses marked EVAP, available from your local automotive supply store. If you suspect a problem with the purge valve or any other component, see Section 5 for information on testing the system.

On 1996 and later vehicles, if a problem occurs with the EVAP system, the Check Engine light will be activated.

Canister Purge Regulator Valve
  1. With the engine OFF, disconnect the hoses from both valves and connect a clean length of hose. It should be possible to blow through the frequency valve (normally open) but not the solenoid valve (normally closed).
  2.  
  3. With the engine coolant less than 140°F (60°C), disconnect the purge hose from the canister to the frequency valve. With the engine at idle, there should be no vacuum.
  4.  
  5. As the engine warms to operating temperature, the solenoid valve should be ON and open. The frequency valve should begin to cycle ON and OFF , open and closed. There will be strong vacuum for about 30 seconds, then little or no vacuum for about 60 seconds.
  6.  
  7. If the system does not perform as described, pull back the connector boots so a voltmeter or test light can be connected with the wiring still connected to the valves. If there is no voltage, the wiring or the engine control unit may be faulty. If voltage appears at the intervals described, 1 or both of the valves is faulty. Using a suitable Digital Multi-Meter (DMM) check the resistance of the purge regulator valve as follows:
  8.  
  9. Disconnect the EVAP canister purge regulator valve harness electrical connector.
  10.  
  11. Measure the resistance at the electrical terminals of the EVAP canister purge regulator valve. The resistance should be 20-28 ohms.
  12.  
  13. If out of specification, replace the EVAP canister purge regulator valve.
  14.  



Click image to see an enlarged view

Fig. The EVAP purge regulator valve is typically located near the air filter housing-1.8L Passat model shown

Charcoal Canister And Check Valves
Digifant System
  1. Disconnect the top hose from the valve and connect a hand vacuum pump to the small port on top of the valve.
  2.  
  3. Disconnect the hose from the carbon canister to the control valve and run the engine at idle to provide vacuum to the bottom port.
  4.  
  5. Place a finger over the open port of the valve. When there is vacuum at the bottom port, you should not feel any vacuum at the open port.
  6.  
  7. Draw a vacuum on the small top port. The valve should open and you should feel vacuum at the open port.
  8.  

Motronic System
  1. With the engine OFF, disconnect the hoses from both valves and connect a clean length of hose. It should be possible to blow through the frequency valve (normally open) but not the solenoid valve (normally closed).
  2.  
  3. With the engine coolant less than 140°F (60°C), disconnect the purge hose from the canister to the frequency valve. With the engine at idle, there should be no vacuum.
  4.  
  5. As the engine warms to operating temperature, the solenoid valve should be ON and open. The frequency valve should begin to cycle ON and OFF , open and closed. There will be strong vacuum for about 30 seconds, then little or no vacuum for about 60 seconds.
  6.  
  7. If the system does not perform as described, pull back the connector boots so a voltmeter or test light can be connected with the wiring still connected to the valves. If voltage appears at the intervals described, 1 or both of the valves is faulty. If there is no voltage, the wiring or the engine control unit may be faulty.
  8.  

 
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